Iron-based superconductors respond well to pressure

Superconductivity—the ability of certain materials to conduct electricity with no resistance—continues to be one of the most challenging fields in materials science. On the one hand, the effect appears reliably in a number of materials, although only at very low temperatures. Those temperatures went up with the discovery of copper-based (cuprate) superconductors in 1986. In the last four years, iron-based superconductors have been developed and seen the maximum temperature of their superconducting transition pushed higher, although it's still cold compared to both the cuprates and room temperature. But the exact way in which these superconductors perform their tricks is still unclear.

One form of iron-based superconductors, the chalcogenides, are very unusual, since they are strongly magnetic—in other superconductors strong magnetism destroys the effect. ("Chalcogenide" is pronounced with a hard "ch" as in "chemistry", and refers to the presence of the chalcogenide element selenium.) Now, a new report in Nature indicates that they have another unusual property: high pressures, which normally kill superconductivity, can cause them to undergo a phase transition that not only restores the behavior, but raises the critical temperature.

High-critical-temperature (high-Tc) superconductors have transitional temperatures greater than 30K (so "high" is a relative term). Several iron-based superconductors have been studied since 2008, while the first studies of iron chalcogenide superconductors (containing selenium, along with some combination of potassium, thallium, cesium, or rubidium) were published in 2011. The strong magnetic properties of the chalcogenide superconductors have make them intriguing, as scientists currently have no good explanation for the effect.

Superconductivity is primarily a function of temperature: below a certain critical temperature, some materials have zero resistance to electric current; above the transition, they behave as an insulator or some other more normal material. The critical temperature can be raised or lowered by controlling other factors: the addition of impurity atoms (dopants) that alter the electronic structure, subjecting the material to magnetic fields, or applying pressure.

Pressure is a useful parameter to adjust, since doing so doesn't alter the chemical composition of the material. As with temperature, there is a maximum pressure at which superconductivity can occur; plotting the dependence of the critical temperature and pressure (or other parameters) on each other produces a phase diagram for the material.

In the current study, Liling Sun et al. discovered that applying very high pressure to certain chalcogenide superconductors (Tl0.6Rb0.4Fe1.67Se2, K0.8Fe1.7Se2, and K0.8Fe1.78Se2) produced a new phase of superconductivity rather than destroying it.

These materials normally have critical temperatures in the neighborhood of 30-32K. As expected from previous experiments, the researchers found that applying pressure to it caused the superconductivity to drop until it vanished entirely at around 10 billion Pascals (10 GPa, or about 100,000 atmospheres). However, at even larger pressures (13.2 GPa, or 130,000 atmospheres), superconducting behavior reappeared abruptly. And, perhaps even more surprisingly, it reappeared with a higher critical temperatures, around 48.0-48.7K. Above 13.2 GPa, the material suddenly ceased superconducting again.

Such bizarre behavior has never been seen in other materials, and the authors do not speculate about what is causing it. Since superconductivity in iron chalcogenides isn't well understood in general, such caution seems wise. However, further experiments may reveal the missing details needed to provide full theoretical understanding of these materials, both of the anomalous magnetism and the strange new superconducting phase arising at high pressures.

14 Reader Comments

yea, 1.45 million PSI... that's how we should proceed to making superconducting computers?

I'm stoked that we're finding new ways to manipulate materials for superconductivity, but having to put 1.45 million PSI on the material to get there while also at 45K is still only going to be useful in a lab.

The frustrating thing is that we still don't have a workable theory for *why* cuprates superconduct. If we did, we could potentially improve upon them without stumbling along blindly. The fact that we have another whole class of superconductors with no theory to explain or unite them with cuprates is maddening.

Do we understand the underlying physics well enough to construct computer models? If so, what is preventing modeling research into superconductors? If not, what do we need to understand still (if this can even be defined)?

The difficulty in getting a good theory of superconductors, and in particular the cuprates, is that it's really a quantum-mechanical many body problem. If you thought it was hard in newtonian mechanics...

The standard BCS theory of (low critical temperature, including type II) conventional superconductivity basically relies that in certain metals there is an attractive interaction between electrons. This wasn't obvious at all, and in fact really needed the development of QFT before it could be understood.

In high-Tc materials we are reasonably confident that there's some kind of attractive interaction between electrons, the problem is in figuring out where the bloody thing comes from. Magnetic interactions are a definite possibility: most cuprates and iron pnictides are antiferromagnets at higher temperatures. This is why results like this are so interesting for theorists -- they could hint at the microscopic origin of the electron-electron interaction. This is all complicated by the fact that these materials are very anisotropic and full of broken symmetries, and that the interaction (probably) depends strongly on the direction electrons are moving in the material.

In short, while people do do a lot of work (including modeling! check out cond-mat.supr-con on the arXiv!) on this sort of thing, there's just so much we don't know about the microscopic physics in the materials that it's not clear how to more forward! Very smart people have been arguing about this for 20 years, and it's likely to require significant advances in the QM of many-particle systems for any real resolution...

Personally, I disagree with the author's statement that pressure normally supresses superconductivity. The behaviour is nontrivial. The classical BCS example, aluminium, does have lower Tc at higher P. However, other elements such as Li and K see increased Tc with P. In the case of Li, this effect is pretty dramatic, going from a few microkelvin at ambient pressure to over 10 K at pressures similar to this paper. Of course, Lithium has some very interesting eleclronic behaviour going on, but this is why it's nontrivial. In any case,

If you take a look at this image (credit N. W. Ashcroft, Nature 419, 569 (2002), found using google)https://efree.gl.ciw.edu/sites/efree.gl ... ucting.jpgThere are more orange elements (superconducting only under pressure) than pink ones (superconducting at ambient pressure). Note that list is outdated.

Finally, I'd say that the normal state of a material when T>Tc is metallic. OK, so cuprates are basically doped Mott insulators. Well, typical BCS, pnictide and chalcogenide superconductors are metallic. Also, BCS superconductivity is based in the coupling of electronic states on the Fermi surface. No Fermi surface, no superconductivity. Having one means the system is metallic.

A final remark: Hydrogen is predicted to have a Tc of 240 K at pressures about 15 times higher than this paper.

A final remark: Hydrogen is predicted to have a Tc of 240 K at pressures about 15 times higher than this paper.

While that pressure may be as practical as a fart in a hurricane, would not that K put it in around what can be achieved with your average freezer? Err, never mind. Did the actual conversion and it lands on -33c. Still colder than most but at least in a more "practical" range.